The invention relates to a transmission device of variable power. More particularly, the invention pertains to a transmission device having a very wide range of power variation.
Within the framework of wide-bandwidth wireless networks, it is known practice to use ever higher frequencies in order to allow the transmission of high bit rate applications such as video. These networks are also intended to allow data exchanges between at least one base station and a plurality of subscribers. These networks are commonly referred to as point-multipoint networks.
The point-multipoint type radio transmission systems are known to the person skilled in the art by the acronyms MMDS (standing for Microwave Multipoint Distribution System), LMDS (standing for Local Multipoint Distribution System) and MVDS (standing for Multipoint Video Distribution System). These systems used for the broadcasting of programmes permit a return path to the subscriber terminals which allows the subscriber to interact with the programme received. At present, the MPT-1560-RA standard provides for the use of frequencies situated between 40.5 and 42.5 GHz.
FIG. 1 illustrates an LMDS type system. A broadcasting station 1 furnished with a transmission/reception antenna 2 broadcasts information destined for a plurality of subscribers 3. Each subscriber 3 has an external unit 4, consisting of an antenna and means for transposing the signal received or the signal to be transmitted to an intermediate frequency, and an internal unit 5 which comprises means of channel selection in transmission and in reception as well as various means of coding/decoding for exchanging data with at least one user apparatus 6, for example a television set, a telephone or a personal computer.
Transmissions in millimetre bands are strongly dependent on time, on atmospheric conditions, and on the distance D separating the subscriber 3 from the broadcasting station 1. FIG. 2 represents the attenuation to a 40 GHz (giga Hertz) transmission as a function of distance. Curve 7 represents the attenuation in good weather and curve 8 represents the attenuation during rain. As may be observed by the person skilled in the art, the attenuation varies by around 50 dB as the distance varies from 50 m to 2 km.
Now, a base station is used for several subscribers and the subscribers must transmit with the least power so as not to saturate the reception of the base station and jam the other subscribers. It is therefore necessary to have an adjustment of the transmission power level for each subscriber. A dialogue between the broadcasting station and the device placed at the subscriber""s home makes it possible to adjust the transmission power.
FIG. 3 represents an exemplary embodiment of the transmitter part of the device placed at the subscriber""s home, according to the state of the art. The internal or first unit 5 comprises means of modulation 10 which receive a signal to be modulated. A mixer 11 is connected to the modulation means 10 so as to transpose the modulated signal into an intermediate frequency band, a voltage-controlled oscillator 12 delivering the transposition signal. The output of the mixer 11 is coupled to an amplification facility which comprises one or more amplifiers 13 and a variable-gain attenuation circuit 14. A supervisory circuit 15 sends the various control signals to the modulation means 10 to the controlled oscillator 12 and to the attenuation circuit 14. A coaxial cable is connected at the output of the amplification facility so as to transport the signal to the external or second unit 4. The external unit 4 comprises a first filter 16 connected to the coaxial cable and to a mixer 17. A local oscillator 18 delivers a transposition frequency to the mixer. A second filter 19 eliminates the image frequencies originating from the mixer 17. An amplifier of SSPA type (standing for Solid State Power Amplifier) amplifies the signal before delivering it to an antenna 21.
In the circuit of FIG. 3, the adjustment of the transmission gain is carried out in the internal unit 5 with the aid of the attenuation circuit 14. One problem is that the adjustment of the power with the aid of the attenuator 14 alters the signal-to-noise ratio. It is not possible to have more than 35 dB of power variation if one wishes to comply with a signal-to-noise ratio of at least 25 dB.
The invention proposes a transmission path structure which makes it possible to increase the range of variation of the transmission power without degrading the signal-to-noise ratio. The attenuation is carried out in part in the internal unit and in part in the external unit by switching an amplifier.
In a first inventive arrangement a transmission device comprising: an internal unit which modulates data to be transmitted and transposes them into an intermediate frequency band and which comprises means of variable attenuation; an external unit which transposes at least one intermediate band signal into a transmission frequency band, the said external unit comprising at least one amplifier linked to an antenna; and a linking cable which links the external unit and the internal unit. The external unit comprises a switching circuit arrangement for coupling the amplifier output to the antenna or coupling the amplifier input to the antenna.
In a second inventive arrangement a transmission/reception device comprising: an internal unit which, on the one hand, modulates data to be transmitted and transposes them into an intermediate frequency band and which comprises means of variable attenuation, and, on the other hand, transposes and demodulates data received; an external unit which transposes at least one intermediate band signal into a transmission frequency band and at least one signal received in a reception band into the intermediate band, the external unit comprising at least one amplifier linked to an antenna for the transmission of data; a linking cable which links the external unit and the internal unit. The external unit comprises a switching circuit for selectably coupling or bypassing the amplifier to the antenna.
According to a particular embodiment, the internal unit comprises means for delivering a switching signal to the external unit and the external unit comprises means for receiving the switching signal and controlling the switching circuit. This embodiment permits the control of switching in the external unit by the internal unit.
Preferably, the means of variable attenuation make it possible to have a greater range of variation of attenuation than the amplification change achieved by the amplifier bypass switch. Thus, it is possible to switch the external unit between two signal bursts without impeding transmission.